This invention relates to a process for preparing tetrakis(trimethylsilyl)silane which is useful as a CVD material for amorphous silicon carbide film and an intermediate to form a variety of functional materials, and tris(trimethylsilyl)silane which is useful as a reducing agent and a hydrosilylating agent.
Tetrakis(trimethylsilyl)silane is traditionally prepared by reacting tetrachlorosilane with chlorotrimethyl-silane in the presence of lithium metal. See (1) Journal of the American Chemical Society, 86, 1451 (1964) and (2) Journal of Organometallic Chemistry, 8, 245-253 (1967).
The methods in these reports (1) and (2), however, use lithium metal in large excess. Since a large amount of activated lithium metal is inevitably left in the reaction system, the residual lithium metal must be removed therefrom in the work-up by filtration. This operation is not only cumbersome, but also very hazardous and undesirable for the industrial manufacture because ignitable fine lithium metal must be filtered.
To avoid such hazards, a method for treating the residual lithium metal by suitable means other than filtration is needed. It was contemplated to feed the reaction mixture containing lithium metal to another reactor charged with a compound having active proton(s), e.g. water, capable of reacting with lithium metal for deactivation, thereby decomposing the residual lithium metal. This method, however, requires two reactors and a feed line for transferring the lithium metal dispersion from one reactor to the other, resulting in a complex manufacturing installation. It is difficult to completely eliminate the danger that the metallic lithium dispersion ignites during transfer, which makes this method also undesirable for the industrial manufacture.
To obtain tetrakis(trimethylsilyl)silane in high yields by the methods in the reports (1) and (2), a large excess of lithium metal is used as mentioned above. A method adding water or alcohol at the end of the reaction is safe and simple, but the system becomes alkaline during addition, resulting in decomposition and a drastically reduced yield of the desired product.
The report (2) also describes a method employing an amount of lithium metal less than theoretically required in the reaction in order to prevent the lithium metal from remaining. This method, however, leads to a drastic decline of yield, and the reaction system becomes a suspension of lithium chloride and other by-products on which adsorption occurs. It is thus very difficult to consume the lithium completely.
An object of the invention is to provide a process for preparing tetrakis(trimethylsilyl)silane on an industrial scale by reacting tetrachlorosilane with chlorotrimethylsilane in the presence of lithium metal, and by treating the residual lithium metal in a safe and simple manner without lowering the yield. Another object of the invention is to provide a process for preparing tris(trimethylsilyl)silane from the tetrakis(trimethylsilyl)silane thus obtained.
It has been found that after tetrachlorosilane is reacted with chlorotrimethylsilane in the presence of lithium metal to form tetrakis(trimethylsilyl)silane, a compound having active proton(s), e.g. water, is added to the reaction mixture while keeping the mixture neutral or acidic, whereby the residual lithium metal can be treated in a safe and simple manner without a need for filtration of lithium metal or transfer of a lithium-containing suspension to another reactor. It has also been found that by reacting thus obtained tetrakis(trimethylsilyl)silane with an alkyl lithium or alkali metal alkoxide and by hydrolyzing the intermediate with an acid, tris(trimethylsilyl)silane can be prepared in a safe and simple manner.
The invention provides a process for preparing tetrakis(trimethylsilyl)silane comprising the steps of reacting tetrachlorosilane with chlorotrimethylsilane in the presence of lithium metal, adding a compound having active proton(s) to the reaction mixture for treating the residual lithium metal therewith while maintaining the mixture neutral or acidic, and separating tetrakis(trimethylsilyl)silane from the resulting organic layer. Reaction of the tetrakis(trimethylsilyl)silane thus obtained with an alkyl lithium or alkali metal alkoxide, followed by acid hydrolysis, affords tris(trimethylsilyl)silane. This process for preparing tris(trimethylsilyl)silane is another embodiment of the invention.
When tetrachlorosilane is reacted with chlorotrimethylsilane in the presence of lithium metal, there can be employed any of procedure (A) of adding dropwise chlorotrimethylsilane to a reactor charged with lithium metal and tetrachlorosilane, procedure (B) of adding dropwise tetrachlorosilane to a reactor charged with lithium metal and chlorotrimethylsilane, and procedure (C) of adding dropwise tetrachlorosilane and chlorotrimethylsilane to a reactor charged with lithium metal. However, at a high concentration and high temperature, both tetrachlorosilane and chlorotrimethylsilane can react with lithium metal to form undesirable products, for example, silicon-containing oligomers and polymers resulting from tetrachlorosilane and hexamethyldisilane resulting from chlorotrimethylsilane. Therefore, procedure (C) allowing for reaction at nearly room temperature is preferable rather than procedures (A) and (B) wherein reaction must be carried out at a low temperature or low concentration.
The reaction can proceed in a solventless system although it is preferred to use aprotic solvents, for example, ether solvents such as tetrahydrofuran (THF) or diethyl ether and hydrocarbon solvents such as pentane or hexane. Reaction conditions including temperature may be the same as the known reaction conditions.
After the reaction, the residual lithium metal is treated according to the invention by adding a compound having active proton(s) to the reaction mixture while maintaining the mixture under neutral or acidic conditions, preferably at or below pH 7.
The procedure of treating the residual lithium metal while always maintaining the reaction mixture neutral or acidic can be carried out in several ways. In one procedure, a substantially water-free acid is added to the reaction mixture as the compound having active proton(s). The acid used herein may be selected from inorganic acids such as hydrogen chloride and hydrogen bromide and organic acids such as formic acid, acetic acid, methanesulfonic acid, trifluoromethanesulfonic acid, and benzenesulfonic acid.
In another procedure, a compound which does not directly react with lithium metal, but can react with active proton(s) to generate an acid or instantaneously neutralize lithium hydroxide or lithium alkoxide, typically an acid anhydride (such as acetic anhydride) or an acidic oxide (such as diphosphorus pentoxide), is added at the end of reaction, and a compound with active proton(s) is then added. The compound having active proton used herein may be selected from water, alcohols such as methanol, inorganic acids such as hydrochloric acid, hydrobromic acid, and sulfuric acid, and organic acids such as formic acid and acetic acid.
Alternatively, reaction can be carried out under the condition that the overall chlorine equivalent of tetrachlorosilane and chlorotrimethylsilane used in the reaction is equal to or greater than the equivalent of lithium metal, a compound with active proton(s) being added to the reaction mixture at the end of the reaction. In this procedure, the respective amounts of tetrachlorosilane and chlorotrimethylsilane used are arbitrary as long as the overall chlorine equivalent of tetrachlorosilane and chlorotrimethylsilane used in reaction is at least equal to the equivalent of lithium metal. With the yield and economy taken into account, when 1 mol of tetrachlorosilane (corresponding to a chlorine equivalent 4 mol) is used, it is preferred to use 4 to 8 mol of chlorotrimethylsilane (corresponding to a chlorine equivalent 4 to 8 mol). In this case, the overall chlorine equivalent is 8 to 12 mol, the amount of lithium metal used is an arbitrary amount not greater than the overall chlorine equivalent, that is, 8 to 12 gram-atom, and preferably at least the theoretical amount for one mol of tetrachlorosilane, that is, 8 gram-atom. The compound having active proton(s) used herein may be selected from water, alcohols such as methanol, acids such as hydrochloric acid, and any other compounds having active proton(s).
Among the above-mentioned procedures of always keeping the reaction mixture neutral or acidic, the last-mentioned procedure is most simple and preferable.
The amount of the compound having active proton(s) added should be enough to remove the residual lithium metal, usually 1 to 50 times, and preferably 1 to 10 times the molar amount of residual lithium metal.
After metallic lithium has been treated, the desired tetrakis(trimethylsilyl)silane is separated from the resulting organic layer by any method. Concentration of the organic layer followed by addition of an alcohol such as methanol thereto for crystallization typically affords tetrakis(trimethylsilyl)silane as a solid, which may be further purified by recrystallization and/or sublimation.
By cleaving a silicon-silicon bond of the tetrakis(trimethylsilyl)silane thus obtained, tris(trimethylsilyl)silane can be prepared.
The process of preparing tris(trimethylsilyl)silane by silicon-silicon bond cleavage may be conducted in accordance with well-known procedures, for example, Org. Synth., 70, 164-168 (1992) and Eur. J. Inorg. Chem., 221-226 (1998). In one preferred procedure, tetrakis(trimethylsilyl)silane is reacted with an alkyl lithium or alkali metal alkoxide and the reaction product is hydrolyzed with an acid.
More illustratively, tris(trimethylsilyl)silane can be prepared by adding an alkyl lithium such as methyl lithium or butyl lithium to a solution of tetrakis(trimethylsilyl)silane in a solvent to effect reaction, adding the reaction solution dropwise to an acid to effect hydrolysis, separating, concentrating and vacuum distilling the organic layer. The amount of alkyl lithium used herein is usually 1 to 1.5 equivalents relative to tetrakis(trimethylsilyl)silane, and the reaction temperature is usually ambient temperature in the range of 10 to 30xc2x0 C. The solvent used herein is selected from ether solvents such as tetrahydrofuran and diethyl ether, hydrocarbon solvents such as hexane and pentane, and mixtures thereof. Use of ether solvents is preferred in consideration of the solubility of alkyl lithium. For hydrolysis, there may be used inorganic acids such as hydrochloric acid and sulfuric acid, organic acids such as formic acid and acetic acid, and aqueous solutions thereof. The amount of acid used is preferably 1 to 50 times, more preferably 1 to 5 times the molar amount of alkyl lithium. During hydrolysis, the reaction temperature is preferably kept at or below room temperature.
The tetrakis(trimethylsilyl)silane obtained by the process of the invention can also be converted into tris(trimethylsilyl)silane by the procedure described in Eur. J. Inorg. Chem., 221-226 (1998). More illustratively, tris(trimethylsilyl)silane can be prepared by adding an alkali metal alkoxide such as potassium tert-butoxide, sodium tert-butoxide, potassium methoxide or sodium methoxide to a solution of tetrakis(trimethylsilyl)silane in a solvent to effect reaction, adding the reaction mixture dropwise to an acid to effect hydrolysis, separating, concentrating and vacuum distilling the organic layer. For this reaction, ether solvents and hydrocarbon solvents can also be used although use of dimethoxyethane and tetrahydrofuran (THF) is preferred because the alkali metal alkoxide is most soluble therein. The amount of alkali metal alkoxide used herein is preferably at least 1 equivalent, more preferably 1 to 1.1 equivalents relative to tetrakis(trimethylsilyl)silane, and the reaction temperature is usually ambient temperature in the range of 10 to 30xc2x0 C. For hydrolysis, there may be used inorganic acids such as hydrochloric acid and sulfuric acid, organic acids such as formic acid and acetic acid, and aqueous solutions thereof. The amount of acid used is preferably 1 to 50 times, more preferably 1 to 5 times the molar amount of alkali metal alkoxide. During hydrolysis, the reaction temperature is preferably kept at or below room temperature.
By using the tetrakis(trimethylsilyl)silane obtained by the process of the first embodiment as a starting material for tris(trimethylsilyl)silane, the latter can be prepared in a safe and simple manner. This tris(trimethylsilyl)silane is useful as a reducing agent, hydrosilylating agent and so on.